Extraction of Kerogen from Oil Shale (Puertollano, Spain) - American

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Ind. Eng. Chem. Res. 2011, 50, 1730–1738

Extraction of Kerogen from Oil Shale (Puertollano, Spain) with Supercritical Toluene and Methanol Mixtures M. C. Torrente and M. A. Galań* Chemical Engineering Department, UniVersity of Salamanca, Plaza de Los Caı´dos, 1-5, 37008 Salamanca, Spain

The solubility of kerogen from Puertollano (Spain) oil shales in supercritical mixtures of toluene and methanol with 20, 40, 60, and 80 mol % methanol were studied in a range of temperatures and pressures of 603-623 K and 7.0-20.0 MPa, respectively. The bitumen obtained with each of the solvents and operation conditions was analyzed by gas chromatography coupled with a mass detector. The results indicate that the composition of the bitumen is dependent on the supercritical solvent and independent of the operation conditions. Solubility data were correlated with the Chrastil equation. This model correlates the experimental data properly when the conditions are distant from the critical point. From this equation, the solvation parameter k was found to be between 0.9 and 1.5 for different mixtures of toluene and methanol. Singular values of k were obtained for mixtures with 20 and 40 mol % methanol. This is probably due to a certain molecular aggregation between the toluene and methanol, perhaps an azeotrope, at those concentrations and operation conditions. Introduction Dependence on petroleum could be reduced by using other fossil hydrocarbon resources, such as coal, oil shales, and tar sands. One of the advantages of these sources is that their reservoirs are 4 or 5 times larger than proven petroleum deposits. However, their exploitation is not straightforward and several problems arise, such lower calorific power, greater environmental impact, and more expensive mining costs. Thus, new and more efficient technologies must be developed. Oil shales are sedimentary rocks composed of detrital grains embedded within an inorganic matrix that contains an insoluble organic matter called kerogen (C200H300SN5O11). The decomposition of this kerogen produces bitumen, which is soluble in organic solvents.1 Classically, pyrolysis has been used to extract kerogen from oil shales. This process requires the treatment of large amounts of solid material, including a method for grinding the rocks down to a suitable particle size.2 Here, in an attempt to test processes other than pyrolysis, a technology based on the use of solvents under supercritical conditions (SC) was used. Several studies addressing the collection of bitumen from oil shales and tar sands have been carried out with propane,3-5 toluene,6-9 and water10-12 under supercritical conditions. In those studies, some advantages were reported such as better selectivity, the possibility of using a lower working temperature, a more efficient energy balance and better extraction capacity: 100% vs 60-70% in pyrolysis.8 Despite these advantages, the most important drawback to develop this process is the low solubility of bitumen and a lack of thermodynamic equilibrium data. In a previous work,13 the solubility of bitumen was studied using pure toluene and methanol as extractants under SC with a range of reduced temperatures and pressures of 1.02 to 1.21 and 1.02 to 3.85, respectively. It was found that when toluene was used as solvent, the organic material of oil shale was depleted and mainly aromatic compounds of kerogen were extracted, with a small portion of paraffinic hydrocarbons. * To whom correspondence should be addressed. Tel.: +34-923294479. Fax: +34-923-294574. E-mail: [email protected].

Moreover, when methanol was used, linear, saturated, and unsaturated hydrocarbons were obtained, with a very small proportion of aromatic compounds. From that study, it was concluded that the composition of the extracted bitumen was strongly dependent on the solvent but independent of operation conditions, at least within the range of temperatures and pressures used.13 This is in agreement with other studies addressing the SFE of carbon.14-17 To confirm this effect, the main aim of the present work is to study the supercritical extraction of kerogen from oil shales by using mixtures of toluene-methanol as solvents, increasing the percentages from 20 to 80 mol % methanol, in order to analyze the composition of bitumen and the extraction capacity of the mixtures. The operation conditions were in the 603-623 K and 7.0-20.0 MPa ranges. Solubility data must be correlated in order to predict phase behavior. To accomplish this, many semiempirical models, very useful because of their simple solutions, have been developed. Among them, the one most widely used is the Chrastil’s model,23 which allows the solubilities of liquids and solids in supercritical gases to be calculated. According to this model, solubility is directly related to solvent density. This density can be determined by solving an equation of state (EoS) and mixing rules. There are many equations of state and mixing rules defined in the literature,18-22 and it is not easy to decide which one should be applied to fit the data. In the present work, Chrastil’s model was used owing to its simplicity and the reasonable fitting of the data achieved, together with Peng-Robinsong (PR) and Soave-Redlich-Kwong (SRK) equations of sate and the Huron-Vidal (HV) mixing rule to determine density. Experimental Section The oil shale used was from Puertollano, Ciudad Real (Spain) and has been characterized in previous contributions.2,24 The original shale was milled and sieved: sizes of 1.00-1.41 (18-14 Mesh) were used. The oil shales were dried in an oven at 383 K and fluidized to avoid dust. They were kept in a dissecator. The extraction fluids were mixtures of 99.5 mol % toluene, with supercritical constants: Tc ) 591.8 K; Pc ) 4.10 MPa, and 99.8 mol % methanol: supercritical constants: Tc ) 512.6

10.1021/ie1004509  2011 American Chemical Society Published on Web 12/27/2010

Ind. Eng. Chem. Res., Vol. 50, No. 3, 2011

Figure 1. Diagram of equipment.

K; Pc ) 8.09 MPa. The mixtures were prepared by the addition of methanol to toluene up a composition of 20, 40, 60, and 80 mol % of methanol.

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The experimental device (Figure 1) and methods used have been described elsewhere.13 Solubility data were obtained according to Brunner’s method, called the “analytic static method”.25 Following this method, a weighed amount of oil shale was placed in the reactor with different volumes of toluene/methanol mixtures at different temperatures and pressures. When equilibrium had been reached, samples of bitumen were withdrawn. For each temperature, the pressure reached in an experiment was a function of the solvent volume loaded into the reactor. The equipment consisted of two identical pressure-resistant reactors (Figure 1), one with a capacity of 1 L and the other of 0.5 L (reactor models Parr 4571 and Parr 4575, respectively) with an oven, a temperature controller (Parr Controller 4842), and a pressure sensor (model HD 8804, Delta Ohm). Once the appropriate temperature and pressure conditions had been reached, stirring was maintained for 5/6 h, which was sufficient time for equilibrium to be achieved. Two stainless-steel sample flasks (10 mL volume) were used to collect samples from the top and the bottom of the cell and these samples were analyzed twice to have reproducible data, as described previously. Also, the good agreement between the samples from the top and bottom of the reactor indicated the existence of a single phase in the supercritical phase. To measure the amount of bitumen extracted under the different operation conditions, the samples were analyzed with a spectrophotometer at 440 nm8

Table 1. Most Probable Compounds and Related Areas for Bitumen Extracted with Toluene-Methanol Mixtures at 20 mol % Methanol

most probable compound

% probability

elution time (min)

% area

area correct

formulas

Pm (g/mol)

benzaldehyde benzene, (methoxymethyl)benzenemethanol benzoic acid, methyl ester 1,1′-byphenyl-2,3′ dicarboxylic acid dimethyl ester 1,1′-byphenyl-2,4′ dicarboxylic acid dimethyl ester 1,1′-biphenyl, 2,3′-dimethyl-, 1,1′-biphenyl, 2,2′-dimethyl-, bibenzyl 1,1′-biphenyl, 4,4′-dimethylbenzene, 1-methyl-2-(3-phenylmethyl) methylbenzene, 2,4-dimethyl-1-(phenylmethyl)benzene, 1-methyl-4-(3-phenylmethyl) methylbenzene, 1,1′-methylenebis(4-methyl)SUM

95 97 94 90 97 97 97 96 93 97 98 95 96 97

9.03 10.21 12.15 14.91 31.02 31.52 32.02 32.51 32.92 36.02 36.81 37.37 37.42 37.83

3.79 4.00 2.66 5.49 1.59 1.19 16.49 14.53 23.79 1.87 1.12 1.43 2.06 2.45 84.14

4.50 4.75 3.16 6.52 1.89 1.41 19.60 17.27 28.27 2.22 1.33 1.70 2.45 2.91 100.00

C7H6O C8H10O C7H8O C8H8O C16H14O4 C16H14O4 C14H14 C14H14 C14H14 C14H14 C15H16 C15H16 C15H16 C16H16

106.12 122.17 108.14 136.15 270.28 270.28 182.27 182.27 182.27 182.27 196.29 196.29 196.29 208.30 176.82

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% probability

elution time (min)

% area

area correct

formulas

Pm (g/mol)

benzene, ethylbenzene, 1,3-dimethylbenzene, 1,4 -dimethylbenzaldehyde benzene, methoxymethylbenzenemethanol bibenzyl benzene, 1-methyl-4-(phenylmethyl)benzene, 1-methyl-2-(phenylmethyl)1,1′-biphenyl, 3,4′-dimethyl ester benzene, 1-methyl-2-(4-phenylmethyl) methylbenzene, 2,4-dimethyl-1-(phenylmethyl)SUM

76 94 95 76 96 64 93 98 95 83 90 78

5.74 5.98 6.69 9.03 10.23 12.16 32.91 33.35 33.72 35.66 37.20 37.37

5.60 7.14 3.99 2.10 8.89 1.75 21.00 12.11 10.36 1.12 3.43 2.24 79.73

7.02 8.96 5.00 2.63 11.15 2.19 26.34 15.19 12.99 1.40 4.30 2.81 100.00

C8H10 C8H10 C8H10 C7H6O C8H10O C7H8O C14H14 C14H14 C14H14 C14H14 C15H16 C15H16

106.17 106.17 106.17 106.12 122.16 108.14 182.27 182.27 182.27 182.27 196.29 196.29 159.96

Since the photometer provides the concentration of bitumen in g/L, in order to know the total amount of bitumen extracted, it is necessary to multiply this concentration by the total amount of solvent added to the reactor and thus determine the extraction yield. To check reproducibility, the analyses were repeated at least twice and when the error was higher than 5%, a third analysis was performed. To determine the composition of the bitumen, this was concentrated in a distillation column. Then, the tails of the column were introduced in a vacuum evaporator until almost dry bitumen was obtained. Samples of this bitumen were sent to “SGS Espanõla de Control, S.A.” (a certified external laboratory), where, using methylene chloride as solvent and the standard ASTM-D 2887 method, they were analyzed by gas chromatography and MSD (mass detector) and quantitative data about the composition of the bitumen were obtained. Theoretical Studies To correlate the solubility data, Chrastil’s model was used. According to this model, solubility is directly related to solvent density by the equation: ln c ) k ln d +

( a′T + b′)

(1)

where d is the density of solvent (g/L), c is the concentration of solute in the supercritical phase (g/L); and k, termed the

“associating factor”, is the number of solvent molecules surrounding a molecule of solute. The constants a′ and b′ are defined as follows: a′ )

-∆H R

b′ ) ln(MA + k · MB) + q - k · ln(MB)

(2) (3)

where q is a constant, and MA and MB, are the molecular weights of the solute and supercritical fluid, respectively. ∆H is the total reaction heat, which can be decomposed as follows: ∆H ) ∆Hvap + ∆Hsolv

(4)

where ∆Hvap is the heat of vaporization of the solute, ∆Hsolv is the heat of solvation. In Chrastil’s original paper,23 using the solubility data of various compounds (tripalmitin, R-tocopherol, etc.) in supercritical carbon dioxide and within a temperature operation range of 40 K; the associating factor, k, was considered independent of temperature. The densities (d) of the toluene and methanol mixtures were determined using the Huron-Vidal mixing rule (eq 7) together with the Peng-Robinson (PR),26 and Soave-Redlich-Kwong (SRK)27,28 equations (eqs 5 and 6, respectively) where the covolumes of the mixtures (bm) were calculated following the van der Waals mixing rule (eq 8).


P)

a(T) R·T V-b V · (V + b) + b · (V - b)

(5)

a · TR R·T V-b V · (V + b)

(6)

P)

[∑ n

am ) bm

xi

i)1

bm )

ai g∞E (T, xi) + bi C*

]

(7)

∑xb

(8)

i i

i

P (MPa) and T (K) are the pressure and the temperature of the system, respectively. V (m3/mol) is the molar volume, R (MPa · m3/K · mol) is the gas constant, b (m3/mol) is the covolume, and a (MPa · m6/mol2) is the interaction parameter, which represents the force of attraction between molecules. g∞E (J/mol) is the excess Gibbs free energy to infinite pressure, and C* is a constant, which when the SRK equation is used C* ) -ln 2, whereas for the PR equation C* ) -0.62323. Further details about the calculation of densities from toluene-methanol mixtures are given in Torrente.29 Results As described previously, samples of bitumen were analyzed by chromatography MSD. The objective of the GC-MSD

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analysis was to characterize the bitumen extracted with the different solvents and also to know the molecular weight of the bitumen. From this analysis, the bitumen composition was found to be dependent on the supercritical fluid used in the extraction but not on the operation conditions, in agreement with GHourcade.13 Tables 1-4 show the composition of bitumen extracted with different toluene-methanol mixtures, (20, 40, 60, and 80 mol %). The mass balance of the elements from the bitumen extracted with the different solvents is given in Table 5. Also, the molecular mass of the bitumen extracted with the different supercritical fluids is given in Table 6. From Tables 1-4 it can be observed that the elution time of the same compounds are the same, but there are small differences in the elution time of the isomers. However, this can be considered irrelevant since the aim of the analysis was to know the molecular weight of the bitumen and the structure of extracted compounds, as indicated above. Also, from these tables it may be seen that when the proportion of methanol is increased, the amount of linear hydrocarbons increases. Moreover, oxidized compounds such as alcohols and esters appear. The trend of the amount of these oxidized compounds increased from 7 mol % for pure toluene to more than 50 mol % for pure methanol, with a minimum of about 5 mol % for



% probability

elution time (min)

% area

area correct

formulas

Pm (g/mol)

benzene, methoxymethylphenol, 2-methylphenol, 2,4,6-trimethyltetradecane benzene, hexamethyl1,1′-biphenyl, 2,3′-dimethyl1,2-benzenedicarbolxylic acid dimethyl ester 1,1′-biphenyl, 2,2′-dimethylbibenzyl benzene, 1-methyl-2-(phenylmethyl)benzene, 1-methyl-4-(phenylmethyl)1,1′-biphenyl, 3,3′-dimethyl1,1′-biphenyl, 3,4′ -dimethylbenzene, 1-methyl-2-(3-phenylmethyl)methylbenzene, 1-methyl-2-(4-phenylmethyl)methylbenzene, 1-methyl-2-(2-phenylmethyl)methylbenzene, 1,1′-methylenebis-(4.methyl)SUM

95 97 62 95 93 97 96 95 93 98 98 96 98 96 97 98 97

10.23 13.19 14.74 28.46 30.19 32.02 32.29 32.51 32.93 33.37 33.74 35.66 36.02 36.81 37.19 37.42 37.82

1.39 0.97 0.81 1.49 0.89 1.47 1.39 1.48 17.61 19.58 17.30 1.50 2.05 1.32 1.32 5.52 4.54 80.65

1.73 1.21 1.01 1.85 1.10 1.82 1.72 1.84 21.83 24.28 21.46 1.84 2.54 1.64 1.64 6.85 5.63 100.00

C8H10O C7H8O C9H12O C14H30 C12H18 C14H14 C10H10O4 C14H14 C14H14 C14H14 C14H14 C14H14 C14H14 C15H16 C15H16 C15H16 C16H16

122.16 108.14 136.19 198.39 162.27 182.27 194.18 182.27 182.27 182.27 182.27 182.27 182.27 196.29 196.29 196.29 208.32 183.04

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% probability

elution time (min)

% area

area correct

formulas

Pm (g/mol)

benzene, ethylbenzenemethanol phenol, 2-methylphenol, 4-methyltetradecane 1,4-benzenedicarbolxylic acid dimethyl ester pentadecane bibenzyl benzene, 1-methyl-2-(phenylmethyl)benzene, 1-methyl-4-(phenylmethyl)hexadecane benzene, 1-methyl-2-(3-phenylmethyl)methylbenzene, 1,1′-methylenebis-(4.methyl)SUM

95 97 62 95 93 97 96 95 93 98 96 96 98

5.75 12.15 13.19 14.11 28.46 32.29 32.50 32.91 33.36 33.72 36.36 36.81 37.82

1.61 2.17 2.97 2.80 2.87 20.23 2.03 14.98 11.83 9.80 1.54 4.69 3.71 81.20

1.98 2.67 3.62 3.45 3.53 24.91 2.50 18.45 14.57 12.07 1.90 5.78 4.57 100.00

C8H10 C7H8O C7H8O C7H8O C14H30 C10H10O4 C15H32 C14H14 C14H14 C14H14 C16H34 C15H16 C16H16

106.17 108.14 108.14 108.14 198.39 194.18 212.42 182.27 182.27 182.27 226.45 196.29 208.32 180.67

Table 5. Mass Balance of the Elements from Bitumen Extracted with the Different Solvents 20 40 60 80 toluene methanol mol % mol % mol % mol % mol % carbon mol % hydrogen mol % oxygen

49.9 48.8 0.3

34.1 59.9 6.0

49.1 49.2 1.7

48.2 51.1 0.7

48.9 50.8 0.4

45.2 50.8 4.0

Table 6. Molecular Mass of the Bitumen Extracted with Different Supercritical Fluids toluene methanol 20 mol % 40 mol % 60 mol % 80 mol % molecular mass (g/mol)

176

202

177

157

183

181

the toluene-methanol mixture at 60 mol % methanol. Additionally, an approximate empirical formula for kerogen is C200Ht300SN5O11; this is about 2 mol % oxygen, while the bitumen extracted with methanol has about 6 mol % oxygen. We assumed that this oxygen mainly came from the methanol because an excess of solvent was added to the reactor. All of the foregoing suggests that the extraction mechanism takes place in two steps. First, the solvent passes into the matrix of the oil shale and decomposes kerogen to bitumen, after which some of hydrocarbons that form the bitumen react with the solvent. As the oxygen is in part supplied by the methanol, when the concentration of methanol in the solvent decreases, it is possible that some reactions, the slower ones, do not take place. This

can be the reason causing the absence of some of the products when different solvents (with different methanol concentration) are used. That was suggested because when pure toluene was used as solvent, the amount of aromatic compounds increases in comparison to the products obtained with methanol, which gives place to larger amounts of oxidized products. In agreement with this argument, several authors4,30-32 reported considerable differences in the yields (wt%) and compositions of the oils obtained from oil shale and carbon when solvents of different polarity are used. The minimum of oxidized hydrocarbons can be explained in terms of the fact that part of methanol interacts with toluene, perhaps to form an azeotrope, and this can produce a minimum of oxidized compounds. This is suggested because methanol and toluene form an azeotrope at 88.5 mol % methanol at normal pressure. When the pressure rises the azeotrope probably moves to a lower fractions of methanol, around 20-40 mol %. From all of these results, it may be concluded that the composition of bitumen is only a function of the supercritical solvent and is independent of the operation conditions. The amount of bitumen extracted under the different conditions was measured by photometry at 440 nm. The calibration curve is given in Figure 2; where absorbance is shown vs molar fraction. Since absorbance was measured under normal conditions, to calculate the concentration under the operation conditions it was necessary to correct the solvent density (or its volume). Under


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Figure 2. Photometry calibration curve.

Figure 5. Solubility of bitumen in 20 mol % methanol mixtures. Points are experimental values.

Figure 3. Comparison between extraction agents at 603 K.


Figure 4. Comparison between extraction agents at 623 K.

the working conditions, the maximum volume that the solvent can occupy is the volume of the reactor and under normal conditions, the liquid phase volume is the amount of solvent that is added to the reactor. The concentration of samples was corrected to the total volume of reactor according to the following: Vsolvent

× [C( Lg )] ) (Abs) β V

From Figures 3 and 4, it can be observed that the quantity of kerogen increases with pressure and temperature. This seems to indicate that the effect of the solute vapor pressure is predominant over the effect of solvent density, which decreases when temperature rises. Furthermore, from those figures, it may be deduced that the most efficient solvent for extraction is 100% toluene and the least efficient is methanol, with a 75% kerogen extraction yield. The yield increases when the proportion of toluene in the mixtures is higher (Figures 3 and 4). Several authors have found recovery products greater than 100%.8,33,34 This behavior is observed when the working temperature is greater than the temperature of pyrolysis, although it may be due to a reaction between the solvent and the bitumen. This is logical, despite a possible chemical reaction inside the particles of oil shale, because the large amount of aromatic hydrocarbons containing kerogen makes toluene a better solvent than methanol.35-37 However, as stated before, the composition of bitumen changes with the solvent, ranging from linear to aromatic hydrocarbons, depending on the solvent used. This may be useful, depending on the demands of petrochemical industry.

(9)

reactor

where β is the slope of calibration curve; Vsolvent is the volume of the solvent added, and Vreactor is the reactor volume. Figures 3 and 4 show the weight % of kerogen extracted vs pressure as referred to the total amount of kerogen plus gases: ∼19% of total weight of oil shale using toluene, methanol, and mixtures thereof at 603 and 623 K, respectively.

Discussion The solubility data were correlated using Chrastil’s model. By using the logarithmic form of the Chrastil equation (eq 1) and plotting ln[d] vs ln[C], a straight line was obtained (Figures 5-8). From the slope, parameter k was obtained and both parameters a′ and b′ were found from the intercepts of the straight lines at different temperatures.

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Figure 9. Residuum values vs pressure.

Table 7. In this table, error was calculated by AARD (Average Absolute Relative Deviation) which is defined as follows: AARD )


As can be seen in Figures 5-8, no significant differences were seen when the PR or SRK equations were used and, additionally, the solvation parameter kswithin the experimental errorsdid not vary significantly with temperature, in agreement with Chrastil.23 Nevertheless, in a previous work,13 it was found that when supercritical toluene was used as solvent, k was dependent on temperature. Thus, in order to study a possible associated error by assuming k constant, the residuum minimization method was used. In the residuum minimization method, the difference between the experimental and theoretical concentration of bitumen is calculated according to the following: Residuum ) ln Cexp - ln Ctheoretical ) ln Cexp a′ b′ + + k · ln d T

[(

)

]

(10)

where a′, b′, and k must be calculated to have a minimum rest, which is defined by the sum of the residues calculated for each pressure and temperature. Figure 9 shows the residuum value vs pressure (MPa). It may be noted that for pressures above 6.0 MPa, all of the data are included in residuum values between (0.1; whereas in the case of the data obtained for pressures under 6.0 MPa the error is higher, in agreement with the fact that the Chrastil equation does not function in the critical point neighborhood. This is in agreement with other authors,38-43 who have reported that the model does not work well when the density of the solute is low, which happens at low pressures, close to the critical point. The parameters k, a′, and b′ calculated from the Chrastil equation for the pure solvents and mixtures used are given in

N |ycal - yexp | 1 N 1)1 yexp

∑

(11)

The Chrastil model correlates properly the experimental data because for all of the data the AARD error was smaller than 10%. From Table 7, it can also be observed that the value of the solvation parameter k passes through a minimum for mixtures of 20 and 40 mol % of methanol, with values of 0.9 and 1.0; whereas for methanol, mixtures of toluene-methanol 80, 60 mol %, and pure toluene the values were: 1.2, 1.4, 1.3, and 1.2, respectively. This reinforces the notion of the existence of some type of interaction between methanol and toluene for a composition of approximately 40 mol %, which would modify the interaction between solvent and solute. From Table 7, it can also be observed that b′, a factor associated with the molecular weight of the solute and its boiling point, shows a minimum for mixtures of 20 and 40 mol % methanol, varying from a value of 7.0 for methanol to 16.0 for the mixtures; with 10 for pure toluene. This agrees with the fact that the bitumen extracted with different solvents has different composition, and hence different molecular weights (Table 6). Once the values of a′ and b′ had been obtained, values of ∆H and q were calculated from eqs 3 and 4, respectively. The values of q and ∆H for the different solvents are given in Tables 8 and 9. Table 9 shows that the variation in the enthalpy of the bitumen extraction process from temperatures of 603 K and 623 K is higher for the mixtures than for the pure solvents. The highest values were found when mixtures at 20 and 40 mol % of methanol were used. This variation in enthalpy was greater for mixtures of 20 and 40%, and it seems to confirm the hypothesis, stated previously, of molecular aggregation between toluene and methanol at those concentrations. Since the global variation in enthalpy (∆H) is the resulting sum of the extraction from the original matrix (∆Hvap) and the solvation effect (∆Hsolv), considering that ∆Hvap must be positive and ∆Hsolv is always negative, and since the process is endothermic, it may be deduced that ∆Hvap . ∆Hsolv. Thus, when bitumen is extracted to form a solution with a supercritical solvent, it is necessary to consider not only the


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Table 7. Parameters from the Chrastil Equation Calculated with the Residuum Minimization Method Using the SRK and PR Equations of State for the Calculation of the Densities of the Solvent Mixtures Studied

k a′ (×103) b′ AARD

k a′ (×103) b′ AARD

toluene (exp)

20% (SRK)

40% (SRK)

60% (SRK)

80% (SRK)

methanol (exp)

1.22 ( 0.04 -11.3 ( 0.04 10.4 ( 0.6 3.5%

0.94 ( 0.12 -12.9 ( 1.0 15.4 ( 1.6 6.1%

1.02 ( 0.05 -12.9 ( 0.4 15.1 ( 0.6 2.2%

1.39 ( 0.09 -11.8 ( 0.6 11.4 ( 0.9 3.3%

1.34 ( 0.05 -12.0 ( 0.4 11.9 ( 0.7 2.5%

1.48 ( 0.05 -9.6 ( 0.3 7.0 ( 0.5 7.8%

toluene (exp)

20% (PR)

40% (PR)

60% (PR)

80% (PR)

methanol (exp)

1.22 ( 0.04 -11.3 ( 0.04 10.4 ( 0.6 3.5%

0.91 ( 0.11 -12.9 ( 1.0 15.5 ( 1.7 6.2%

0.99 ( 0.05 -13.0 ( 0.4 15.1 ( 0.7 2.3%

1.35 ( 0.09 -11.8 ( 0.6 11.5 ( 0.9 3.3%

1.30 ( 0.05 -12.0 ( 0.4 12.1 ( 0.7 2.5%

1.48 ( 0.05 -9.6 ( 0.3 7.0 ( 0.5 7.8%

Table 8. The q Parameter in the Chrastil Model solvent mixtures 20% mixtures 40% mixtures 60% mixtures 80% methanol

q (SRK) (SRK) (SRK) (SRK)

14.0 13.9 11.4 11.5 6.6

solvent mixtures mixtures mixtures mixtures

20% 40% 60% 80%

q (PR) (PR) (PR) (PR)

14.0 13.8 11.4 11.5

Table 9. Variation of Enthalpy According to the Chrastil Equation ∆H (×103) (kJ/mol) solvent

T ) 603 K T ) 623 K

toluene 20% (SRK) 40% (SRK) 60% (SRK) 80% (SRK) methanol

56.6 64.7 64.9 59.2 60.1 48.2

58.5 66.8 67.1 61.2 62.1 49.8

∆H (×103) (kJ/mol) solvent 20% (PR) 40% (PR) 60% (PR) 80% (PR)

T ) 603 K T ) 623 K 64.7 64.9 59.2 60.1

66.8 67.1 61.2 62.1

effect required to overcome the attractive forces among the atoms of kerogen (reaction effect), but also the energy that the system requires to remove the bitumen from the shale matrix to the solution (solvation effect). This would explain why the values of ∆H obtained are so elevated. Conclusions Bitumen extraction isotherms were obtained using mixtures toluene and methanol as solvents under supercritical conditions. From the chromatographic analyses it may be deduced that in the process of extraction there are two effects: a breaking of bonds of the kerogen, which gives rise to hydrocarbons of lower molecular weight (bitumen), and a chemical interaction between the solute and solvent. Considering the different solvents used, the most efficient solvent to carry out extraction is toluene and the least efficient is methanol, and when the amount of toluene in the mixtures increases, the quantity extracted also increases. The Chrastil model correlates properly with the experimental data when the operational conditions are distant from the critical point. The solvation parameter k, has a value of 1.2 for toluene to 1.5 for methanol, with a minimum for mixtures of 20 and 40 mol % in methanol of 0.9 and 1.0, respectively. The enthalpy of extraction presents a maximum for mixtures of 20-40 mol % of methanol in a range of temperatures from 603 K to 623 K. All that may be due to the formation of a certain molecular aggregation in the solvent, perhaps to form an azeotrope, for mixtures methanol-toluene of 20-40 mol % methanol at these operation conditions. Nomenclature *a ) Interaction parameter of the cubic equation of state (MPa · m6/mol2).

*a′ ) Constant of the Chrastil equation. *Abs ) Absorbance. *b ) Covolume (m3/mol). *b′ ) Constant of the Chrastil equation. *c ) Concentration of solute in the supercritical phase (g/L). *C* ) Constant of the SRK and PR equations. *d ) Density of solvent (g/L). *g∞E ) Excess Gibbs free energy at infinite pressure (J/mol). *H ) Total reaction heat (J). *Hvap ) Heat of vaporization of the solute (J). *Hsolv ) Heat of solvation (J). *k ) Associating factor. *M ) Molecular weight (g/mol). *P ) Pressure (MPa). *Pc ) Critical pressure (MPa). *q ) Constant of the Chrastil equation. *R ) Gas constant (MPa · m3/K · mol). *T ) Temperature (K). *Tc ) Critical temperature (K). *V ) Molar volume (m3/mol). *Vreactor ) Reactor volume (m3). *Vsolvent ) Volume of solvent added (m3). *β ) Slope of photometry calibration curve. Acknowledgment The financial support of the CICYT (Comisioń Interministerial de Ciencia y Tecnologıá) PPQ2003-04031 and the Junta de Castilla y Leoń SA017A07 under the Outstanding Program (GR30); and ENCASUR, responsible for oil shales distribution, are gratefully acknowledged. Literature Cited (1) Pan, Z. L.; Feng, H. Y.; Smith, J. M. Rates of Pyrolysis of Colorado Oil-Shale. AIChE J. 1985, 31, 721. (2) Torrente, M. C.; Galań, M. A. Kinetics of the Thermal Decomposition of Oil Shale from Puertollano (Spain). Fuel 2001, 80, 327. (3) Deo, M. D.; Hwang, J.; Hanson, F. V. Supercritical Fluid Extraction of a Crude Oil, Bitumen-Derived Liquid and Bitumen by Carbon Dioxide and Propane. Fuel. 1992, 71, 1519. (4) Subramanian, M.; Hanson, F. V. Supercritical Fluid Extraction of Bitumens from Utah Oil Sand. Fuel Process. Technol. 1998, 55, 35. (5) Han, B.; Yang, G.; Ke, J.; Mao, C.; Yan, H. Phase Equilibria of Supercritical Propane-Fengcheng Bitumen System and the Density and Viscosity of the Liquid Phase. Fluid Phase Equilib. 1998, 143, 205. (6) Qin, K. Z.; Wang, R. A.; Jia, S. S. Chemical Structure Investigation of Maoming Oil Shale Kerogen by Supercritical Gas Extraction. Energy Source. Part. A. 1984, 7, 237. (7) Yu¨ru¨m, Y.; Kramer, R.; Levy, M. Thermochemical Reactions in Subcritical and Supercritical Interaction between Mishor Rotem Oil Shale and Toluene. Thermochim. Acta 1986, 105, 51. (8) Triday, J.; Smith, J. M. Dynamic Behaviour of Supercritical Extraction of Kerogen from Shale. AIChE J. 1988, 34, 658.

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ReceiVed for reView March 1, 2010 ReVised manuscript receiVed November 8, 2010 Accepted December 1, 2010 IE1004509

Extraction of Kerogen from Oil Shale (Puertollano, Spain) - American

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